Recently wide use has been found for air-operated reversible percussive action machines, as the provision for reversal is necessary for retracting the machine from holes or for disconnecting the machine from a tube subsequent to driving it into the ground.
An ever increasing application of air-operated reversible percussive action machines requires improvements in their reliability. During the driving of horizontal holes in of lengths over 15-20 meters, or during the driving of vertical holes (such as for deep-hole soil compacting, pressing concrete piles in the ground, etc.) when retracting the machine from such holes is complicated, the mechanism for reversing the percussive action must be especially reliable.
Of no less importance is to the simplification of the device structurally to improve its reliability, make it less complicated and less expensive in fabrication, as well as to ensure a more prompt switchover of the machine to a rearward percussive action.
There is known a device (cf., West German Pat. No. 1,634,417) which includes a substantially hollow cylindrical housing, a hammer accommodated inside the housing for axial reciprocations and delivering impacts on the housing by its front or rear portion, and a stepped tube threadingly connected to the housing through a flange element.
In order to reverse the percussive action of such a device, it is necessary to displace the stepped tube to a new position toward the tail end of the housing by rotating an air supply hose fixedly attached to the stepped tube.
Inherent in the above device is its insufficient reliability because reversal of the percussion action is complicated or even impossible when driving vertical or elongated horizontal holes.
Also, the reversal of the percussive action of the above device requires a number of time-consuming operations affecting its efficiency, these operations including termination of the supply of compressed air, disconnection of the air hose from the compression, and pulling the hose prior to rotating it.
There is also known another percussive action machine (cf., West German Pat. No. 2,340,751) which comprises a hollow cylindrical housing, a hammer reciprocating inside the housing and intended to deliver impacts on the housing by its front or rear portion, and a tube of stepped configuration rigidly secured in grooves of the tail end of the housing through a locking means and connected to the hammer.
For reversing the percussive action of the aforementioned machine use is made, apart from a hose, of a flexible steel cable. By pulling the cable the action of the locking means is eliminated, and the stepped tube is moved to a new position, that is reversal is made possible by a simple linear motion of the stepped tube instead of the rotational movement.
However, the hose must also be turned to bring the stepped tube out of engagement with the grooves and put it in registration with other grooves in its rearmost position. In consequence, the device described in West German Pat. No. 2,340,751 suffers from essentially the same disadvantages as those described heretofore.
There is further known a percussive action machine (cf., West German Pat. No. 2,105,229) which differs from the aforedescribed ones only by the construction of the stepped tube, the latter including an outer step having through radial passages and an inner step with similar through radial passages, the tube being secured coaxially with the housing to be capable of turning and displacing axially relative to the outer step.
A switchover to the reverse percussive action is done here by turning the inner step of the tube relative to the outer step by a multiple supply of compressed air to the machine.
This device offers some advantage over those already described in that is no need for rotating the hose.
However, the machine is disadvantageous in that the multiple delivery of compressed air may bring some uncertainty as to whether the operator is aware of the machine function subsequent to every successive delivery of compressed air, or whether the machine will operate at all, since intermediate positions are possible apart from the forward and rearward percussive actions. Another disadvantage lies in the fact that the machine is structurally overcomplicated and therefore difficult to manufacture.
A device which bears the closest resemblance to one to be proposed in the description that will follow is an air-operated reversible percussive action machine as taught in West German Pat. No. 2,722,298. This machine comprises a housing, a hammer axially and reciprocably disposed inside the housing and having in its tail portion an axial cavity open on one end, and a through radial passage. The hammer and the housing define a front working chamber. A stepped tube also accommodated inside the housing includes substantially cylindrical steps of relatively larger and smaller diameters, with radial passages provided therein.
The larger diameter step is disposed in said axial cavity of the hammer to define a rear working chamber, whereas the step of smaller diameter is secured by a flange in the tail portion of the housing. Installed with precision inside the stepped tube is a spring-loaded sleeve. An air supply hose functions as a means for controlling the axial displacement of the spring-loaded sleeve. the forward percussive action of the machine is initiated when the sleeve assumes the front most of the percussion action is position; reversal accomplished by moving the sleeve to the extreme rear position.
In general, this device obviates the disadvantages inherent in the heretofore described constructions of the prior art percussive action machines, since reversal is achieved by merely applying tension to the hose, and the elements of the machine are at any time in a position to execute either the forward of the rearward actions.
However, even this device is not without some drawbacks. For example, the sliding action and pressure-sealing of the sleeve on two of its cylindrical surfaces requires precise fabrication of the mating surfaces and thus the machine is rather difficult to manufacture. It also lacks reliability because sliding of the sleeve along its two cylindrical surfaces may result in jamming, especially when dirt adheres to such surfaces.
Also, the machine has a pressure-sealed interior consisting of three isolated sections defined by the spring-loaded sleeve and the stepped tube. This interior is likewise made hermetic because of the precision of the sliding surfaces.